Time of Flight Diffraction Technique (TOFD) - An Ultrasonic Testing Method
for all Applications?

Abstract

The article begins with an introduction of TOFD technique principles. The
main part reviews some recent literature wherein excellent results for TOFD
were obtained, especially regarding speed. The author gives examples
of known problems when TOFD is used and recommends not discarding proven test
methods in favor of the cost-saving factor of TOFD, however, TOFD can be a
valuable
add-on for other test methods.

Table of contents

1. Introduction

The first information on Time of Flight Diffraction Technique (TOFD) for
ultrasonic testing on welds was introduced in 1977 [1]. The method was
reported extensively in English publications and was also introduced in
Germany [2]; nevertheless the method was more or less ignored by German
NDT experts. Finally, in 1996, a European pre-standard was announced and
thanks to that and some newly published papers [4-6] it seems that TOFD is
on the
way to replacing radiography and other UT techniques. One paper published
in 1995 [7] referred to the wide acceptance of the method with a
"TOFD Comes of Age" article.

The following main principles describe TOFD:

Two angle beam probes (usual 45°) are placed as a transmitter-receiver
arrangement and are connected together (Fig 1). The distance of the probes
is calculated according to the wall thickness.

Longitudinal waves are usually applied. The sound beam spread is large to
maximize the extent of the scan.

The A-scan (Fig 4) [9] shows the so-called lateral wave, the back wall
echoes and between both signals other signals can possibly appear, which
can occur due to inhomogeneity. The A-Scan is not rectified in the TOFD
technique.

TOFD technique is always applied with imaging methods (Fig 5) [9].

Fig 5 shows the B-Scan image generated by horizontal probe movement and
sound time of flight in a vertical direction. The echo amplitude is displayed
as gray scale, usually zero amplitude light gray (negative maximum amplitude
black, positive maximum amplitude white). For weld testing it is important
to notice that the probes are aligned transversal to the weld, while the
image is generated in the direction of the weld. That means the image
projection of Fig 5 stands perpendicular to the probe projection shown in
Fig 1!

In practice, testing with the TOFD method is only applied by continuously
moving
the probe pair along the weld seam, while in traditional UT techniques the
probe must be also moved perpendicular to the weld seam. Depending on the
equipment the scan is performed either manually or by use of an automated
manipulator. In any case a computerized data evaluation is necessary. In a
very early stage of the TOFD method an instrument called "ZipScan" was
applied, while today many instruments which can perform B-Scans can be used
- many of those are available worldwide.

Three recent examples

A platform in the North Sea was inspected for underwater welds of a
repaired construction with a speed of 45 minutes for each. A radiography
would need 16 to 29 hours [4].

In West Java 2000 m welds on 8 gas containers, the test was carried out at a
very high speed. Every day 60 - 100 m of welds were tested with TOFD /5/.

A report of the Netherlands welding institute (NIL) documented a higher
probability of detection and lower test costs for the TOFD
technique than other
NDT methods [6]. So TOFD is twice as reliable than manual UT and by 1,3
more reliable than radiography. The latter is by 1,5 more expensive than
TOFD.
Besides flaw detection, TOFD can also perform sizing.

Can TOFD perform all NDT tasks?

Does that mean that TOFD is a testing technique which can perform all NDT
tasks?
In the author's opinion the three most important drawbacks of TOFD are
described herein:

1. Sensitivity level

The European pre-standard [3] points out that TOFD only evaluates
the time of flight and not the amplitude of the diffracted echoes.
If the instrument sensitivity (gain) is set on very low level, the TOFD
image would display no diffracted echo.
If the instrument sensitivity is set just above electronic noise level, the
TOFD image will display a lot of diffracted echoes which are caused by
very small inhomogeneities of the weld seam and does not mean that the weld
is really bad.

Also for the TOFD technique it is necessary to define a gain or an
amplitude level because the performed test always
demands acceptance criteria.

2. Crack size determination

The following case is described: A weld was tested during production
according to
AD-HP 5/3 with a sensitivity of detection of FBH 3 mm.
That means that the weld possibly contains many inhomogeneities of FBH 1mm.
In-service by use of the traditional angle beam testing can find a crack.
The same crack can only be detected with a much higher gain setting if the
TOFD
technique is applied, since crack tip echoes respond with a very small
amplitude in a range of FBH < 0,7 mm.

In practice, diffracted echoes at crack tips are not so clear
as they are displayed in Figs 4 and 5.
Crack tip echoes are part of a noise area caused by other irrelevant
diffracted echoes of inhomogeneity.
That can make sizing with the TOFD technique impossible.
A TOFD image inspector needs to perform depiction decisions similar to that
used in radiography. He or she must distinguish the relevant echoes.

3. Detection of small cracks at backside

This is one of the main disadvantages of TOFD. For in-service inspection of
welds it is usually not so important to find old defects inside the weld
seam. More important is the detection of cracks at the backside of
containers or piping.
As an inspection example defects of 0.5 mm depth and app. 10 mm length must
be tested at a pressure component or container of 30 mm wall thickness

The use of diffracted echoes is for that task is not possible. So close to the
back wall
the crack tip echo amplitude is very small.
In that case traditional UT techniques with angle beam
probes and use of the mirror effect must be applied . The TOFD technique is
not applicable here!

Conclusion

Considering the limitations of the TOFD technique described above,
discarding proven test methods in favor of the cost-saving factor of TOFD
is not recommended, however,
TOFD can be a valuable add-on for other test methods.

Let's look at the example of the automated UT of welds for pipelines.
By use of mechanized test systems like ROTOSCAN or PIPECAT it was possible
to replace the radiography method.
The latter uses 8 focused angle beam probes in pulse echo technique in as
in conjunction with one probe pair for TOFD.
Nobody would take the risk of using only the TOFD technique, however it is a
valuable add-on for the complete test.

Of course improvement in the TOFD technique results is possible by
post-processing with the SAFT method. The author does not know if that
application has been applied.

For those who want to read more on TOFD, we suggest:

A literature search via the FIZ-W.

A keyword search (e.g. "tofd") in World Wide Web by use of the search
engines like AltaVista, Lycos, Magellan, etc.
http://www.tabbbsplus.com/other/hottest.htm

A keyword search in Ultrasonic-Testing-Online Journal also in World Wide
Web: http://www.ndt.net

Author

Dr.-Ing Andreas HechtAndreas Hecht is since 1993 in charge of the NDT-unit with a staff of 39 people at BASF´s plant in
Ludwigshafen. He is responsible for the NDT at the plant equipment where all typical
NDT-methods are applied.
Andreas Hecht was born 1953 in Berlin. After his masters degree in material sciences he joined The
German Federal Institute for Materials Testing (BAM), where he worked with ultrasonic
propagation in structured materials and constructed one of the first computer-aided immersion tank
systems with a modular architecture. During his work with BAM he was also involved in the
training of level 3 personnel at the German Society for NDT (DGZfP) as well as training of
NDT-personnel in Indonesia and Kenya.

In 1986 Andreas Hecht finished his thesis on the grain size determination in austenitic sheets by
ultrasonic backscattering and moved from BAM to BASF in Ludwigshafen. At the BASF he was
first in charge of NDT of composite materials and for the development and application of new
scanning inspection-systems before having his present position.
BASF AG
D-67056 Ludwigshafen
Phon: +49-0621-60-56466, Fax: +49-0621-60-54088
Email: